Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum
Recently, device-to-device, D2D, communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency. D2D communication in cellular networks is often defined as direct communication and the mechanisms for controlling such communication as direct control, DC.
Devices that want to communicate directly, or even just discover each other, typically need to transmit various forms of control signalling. One example of such direct control signalling is the so-called beacon signal, which at least carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. Other devices can scan for the beacon signal. Once the devices have detected the beacon, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the beacon.
Multiple devices can transmit control signalling (beacon signals as well as other types of control signalling) simultaneously. The transmissions from the different devices may be time synchronized (mutually time-aligned) or unsynchronized. Synchronization could be obtained for example by receiving appropriate signals from the overlaid cellular network, or from a global navigation satellite system such as GPS. An example of asynchronous beacon reception happens when wireless devices in proximity belong to neighbour unsynchronized cells.
FIG. 4 illustrates one example of DC messages reception in an unsynchronized scenario. The receiver needs multiple, possibly overlapping, reception windows and corresponding parallel FFT processes. Direct control signalling may include DC messages, beacons and the like.
To reduce device power consumption, discontinuous reception, DRX, is typically used. With DRX, the device is sleeping most of the time but regularly (occasionally) wakes up to check for transmissions intended for that device.
Multiple unsynchronized transmissions of control signalling results in several problems:                As the possible time instants when (control signalling) transmissions may occur are not known, each device need to wake up frequently to check for transmissions with a corresponding negative impact on power consumption. This is particularly problematic for beacons, which are expected to be transmitted seldom (with a periodicity in the orders of up to seconds) and which might greatly contribute to discovery latency if their reception is missed.        Reception of multiple unsynchronized and partially overlapping transmissions requires multiple FFTs, adding to the device complexity and is associated to strong inter-message interference and near-far problems.        Multiplexing capacity of multiple transmissions is generally lower in absence of time synchronization.        Additionally, the reception of weak messages might be impossible when strong messages are received on partly overlapping resources in time. This is because the automatic gain control, AGC, at the receiver is typically adjusted based on the strongest signals, and it would be largely not optimal for the weak signals.        
Multiplexing control signalling from multiple devices can be done in multiple ways, for example using Time Division Multiple Access, TDMA, Frequency division Multiple Access, FDMA, or Code Division Multiple Access, CDMA. The choice and/or details of the multiplexing scheme may depend on whether the devices are time synchronized or not. FIG. 1 illustrates an example of TDMA multiplexing of DC messages within a single Direct Control, DC, resource. FIG. 2 illustrates an example of FDMA multiplexing of DC messages within a single DC resource. FIG. 3 illustrates an example of CDMA multiplexing of DC messages within a single DC resource.
Several different transmission schemes for the control signalling can be thought of. One possibility is OFDM and derivatives thereof, e.g. Discrete Fourier Transform, DFT, -precoded Orthogonal Frequency Division Multiplexing, OFDM, which allows for a low-complex yet effective receiver implementation using a Fast Fourier Transform, FFT.